306 



MANUAL OF HISTOLOGY. 



ficiently well stained and transparent to demonstrate the con- 

 stituent parts of myelinic nerve-fibres. The mode of determi- 

 nation depends entirely on the fact that the anterior roots 

 pursue an obliquely descending course through the anterior 

 columns, and for this reason horizontal sections cut the ante- 

 rior rootlets obliquely. (See Fig. 131.) 



What is the natural inference to draw from this fact ? It is 

 this : let the reader look at the upper surface of a transverse 

 section of the spinal cord and bring the anterior roots into the 

 field ; that is, let him look down the anterior column. He 

 readily perceives that the central ends of the anterior root- 

 fibres are nearer his eye than the peripheral ends. He sees 

 that while the central ends are at the focus, the peripheral 

 ends are beyond the focus, and he needs to bring the eye nearer 

 to define them. This nearing the focus also gives the fibre- 

 bundle an apparent peripheral motion, while increasing the 

 focal distance causes an apparent central motion. 



The application of this method to a chance section is easy. 



Suppose we examine the anterior 

 columns of a section and find by 

 focussing that the central ends of 

 the anterior root-fibres are farther 

 from the eye than the peripheral 

 ends. We will immediately know 

 we are looking up 'the cord or^ at 

 the under surface of the section. 

 Now, all it is necessary to do is 

 to turn over the section, either in 

 your mind or on the slide, and put 

 the anterior horns forward. The 

 section is then in position. 



In sections of the cord where 

 the anterior roots do not show, the 

 posterior roots may be used in a similar way, as they, too, pur- 

 sue a slightly descending course. Their use is not so easy, as 

 the fibres are short and pursue a slightly wavy course. In 

 sections or fragments of sections, where neither of these struc- 

 tures avail, a study of the course of the fibres in the anterior 

 white commissure will lead to detection. These fibres pursue 

 a course downward and across the median line, from the base of 

 one anterior horn into the anterior column of the opposite side. 



Fio. 131. Diagram of vertical section 

 of hnman cord through the anterior ami 

 posterior colnmns and the anterior horns. 

 It is intended to demonstrate how a trans- 

 verse, horizontal section. 8, cuts the an- 

 terior nerve-roots obliquely. (From Ar- 

 clUvet of Medicine, August], 1879, p. 70.) 



